In general, compared with a single-antenna cellular system, a MIMO communication system improves in system performance and increases in efficiency of a bandwidth and the like.
Conventionally, a Fractional Frequency Reuse (FFR) technology, an Inter-Cell Interference Coordination (ICIC) technology, and such, have been used as a technology for reducing inter-cell interference. These technologies reduce inter-cell interference by controlling transmit power.
Recently, however, technologies for more effectively controlling interference through more active collaboration between base stations (or Node Bs) have been proposed. In 3rd Generation Partnership Project (3GPP) Long Term Evolution-Advanced (LTE-A) and Institute of Electrical and Electronics Engineers (IEEE) 802.16m, which are two major standard organizations for the next-generation mobile communication, base station collaboration technologies such as a Coordinated Multiple Point Transmission/Reception (CoMP) technology and a Multi-BS MIMO technology are under active discussion, respectively. To reduce inter-cell interference, these base station collaboration technologies include not only a technology of controlling transmit power but also a technology of controlling a spatial domain through antenna processing.
Meanwhile, the CoMP technology used in a downlink includes a Joint Processing (JP) technology and a Coordinated Scheduling/Beamforming (CS/CB) technology. The JP technology is a technology for allowing collaborative base stations to operate as if they are one base station, by sharing traffic data with each other. The CS/CB technology is a technology for allowing collaborative base stations to share scheduling-related information and channel information with each other without sharing traffic data, thereby reducing inter-cell interference.
The JP technology and the CS/CB technology will be described in detail below with reference to FIG. 1.
FIG. 1 illustrates an example of a MIMO communication system using a multi-cell technology.
Referring to FIG. 1, a MIMO communication system includes a first evolved Node B (eNB) 100, a second eNB 110, and first to sixth User Equipments (UEs) 101 to 106.
If the MIMO communication system uses the JP technology, the first and second eNBs 100 and 110, as collaborative eNBs, share traffic data with each other. By sharing traffic data with each other, the first and second eNBs 100 and 110 operate as if they are one eNB. In this situation, all antennas of the first and second eNBs 100 and 110 are used, and data may be transmitted to the first to sixth UEs 101 to 106 via all the antennas.
Alternatively, if the MIMO communication system uses the CS/CB technology, the first and second eNBs 100 and 110 do not share traffic data with each other. Instead, the first and second eNBs 100 and 110 share scheduling-related information, channel information and the like, thereby preventing occurrence of interference between the first and second eNBs 100 and 110.
A Precoder Matrix Index (PMI) restriction technology belonging to the CS/CB technology will be described below with reference to FIG. 2.
FIG. 2 illustrates an example of a MIMO communication system using a PMI restriction technology.
Referring to FIG. 2, a MIMO communication system includes a first eNB 200, a second eNB 210, a first UE 201 performing communication with the first eNB 200, and second to fourth UEs 202 to 204 performing communication with the second eNB 210.
The third UE 203 located in an overlapping cell coverage area between the first and second eNBs 200 and 210, determines whether interference from the first eNB 200 is greater than or equal to a threshold due to use of a precoder. If the interference from the first eNB 200 is greater than or equal to the threshold, the third UE 203 requests the first eNB 200 not to use a PMI that is causing powerful interference to the third UE 203. The third UE 203 transmits the request through the second eNB 210 which is its serving eNB. If the first eNB 200 does not use the PMI in response to this request, interference between a cell of the first eNB 200 and a cell of the second eNB 210 is reduced, contributing to an increase in a Signal-to-Interference and Noise Ratio (SINR) in the third UE 203.
Meanwhile, another technology related to the CS/CB technology includes a PMI recommendation technology. Unlike the above-described PMI technology, the PMI recommendation technology requests a neighbor eNB to use a specific PMI capable of reducing interference, thereby reducing inter-cell interference.
A macro diversity technology and a multi-cell Zero-Forcing Beamforming (ZFBF) technology related to the JP technology will be described below with reference to FIGS. 3 and 4, respectively.
FIG. 3 illustrates an example of a MIMO communication system using a macro diversity technology.
Referring to FIG. 3, a MIMO communication system includes a first eNB 300, a second eNB 310, a first UE 301, and a second UE 302.
If the MIMO communication system uses a macro diversity technology, the first and second eNBs 300 and 310 transmit data only to the second UE 302 out of the first and second UEs 301 and 302, which are located in the same overlapping cell coverage area between the first and second eNBs 300 and 310. That is, in the MIMO communication system including multiple cells, as traffic data is shared between eNBs, only one UE out of multiple UEs that are located in the same overlapping coverage area may receive data at a time. Therefore, when the macro diversity technology is used, spectral efficiency is decreased disadvantageously, but it is possible to reduce interference and increase signal strength desirably because an interference channel is used as an effective signal channel.
FIG. 4 illustrates an example of a MIMO communication system using a multi-cell ZFBF technology.
Referring to FIG. 4, a MIMO communication system includes a first eNB 400, a second eNB 410, a first UE 401, and a second UE 402.
If the MIMO communication system uses a multi-cell ZFBF technology, the first and second eNBs 400 and 410 may transmit data to both the first and second UEs 401 and 402. The first and second eNBs 400 and 410 are adapted to reduce inter-cell interference by using downlink channel information.
If the PMI restriction technology or the PMI recommendation technology belonging to the CS/CB technology among the above inter-cell interference reducing technologies is actually used for a scheduler, various problems may occur. For example, when the PMI restriction technology or the PMI recommendation technology is used, (i) an eNB should determine from which UE it should receive a request to effectively reduce inter-cell interference when a plurality of UEs send different requests, (ii) an eNB should determine from which eNB it should receive a request to effectively reduce inter-cell interference when requests are received from a plurality of eNBs; or (iii) an eNB should determine whether its cell capacity is reduced if it receives a request from an eNB or a UE. Due to the above operation of an eNB, the number of UEs obtaining a gain may be limited.
Meanwhile, when the macro diversity technology among the JP technologies is used, eNBs of a plurality of cells transmit data only to one UE in an overlapping coverage area, making it impossible to acquire higher capacity compared with the single-cell technology in which Multi-User (MU)-MIMO is introduced. When the multi-cell ZFBF technology among the JP technologies is used, high capacity is acquired theoretically, but disadvantageously, the performance may degrade due to the inaccurate channel information and the latency, and the complexity may increase due to the scheduler integration. In order for these JP technologies to be used, because a cluster for inter-eNB collaboration should be fixed, there is a limitation in improving the capacity of a UE located in a boundary of the cluster.